This research aims to further our understanding of how knowledge about the mind develops by investigating five hypothesized acquisitions in children's understanding of the mental state of pretend. Knowledge about the mind is important because it is at the crux of all our social interactions. Our every promise, argument, and assertion is based on the premise that others have minds with which they take in and interpret information. As adults we are so facile with this knowledge that we usually take it for granted, but how it is acquired is not at all obvious. A systematic study of the child's developing comprehension of pretend should be very useful in promoting our understanding of this domain. The mental state of pretend was chosen for this case study because it is of special interest to theorists in this area. Some, for example, have claimed it is the process in which understanding mental representation is first manifest; others have claimed it is the main tool with which social cognitive knowledge is acquired. The proposed research involves conducting a series of cross-sectional experimental studies to test the following five developments in preschoolers' understanding of pretend: (1) that pretend is the province of animate but not inanimate objects; (2) that pretend is intentional in the sense of "done on purpose"; (3) that pretend involves the brain; (4) that mental representation is necessary for pretend; (5) that mental representation is in most cases sufficient for pretend, such that no external manifestation of pretend is required. Note that these proposed acquisitions could be applied to many other mental states as well. However, this is certainly not an exhaustive list of all there is to be learned about pretend nor about mental states generally. These particular developments were chosen because they figure prominently in the existing literature on children's mental state understanding, and yet no systematic investigations have been done charting their acquisition in a single mental state. This area of investigation is relevant to children's health in at least two domains. First, understanding the mind is vital to all our social interactions, and knowing what children understand at different ages is important to clarifying our expectations of them, understanding their vulnerabilities to deception, and so on. A second health-related application of this work concerns autistic children, who have been shown to do very poorly on several tasks entailing a theory of mind. A better understanding of the normal course of understanding mental states like pretend should provide information that will be useful to deciphering the problems underlying autism.